The present invention relates to fiber optic couplers and more particularly to a low-cost method of making fiber optic couplers.
Certain types of fiber optic systems require couplers in which at least a portion of the light propagating in one fiber is coupled to one or more other fibers.
It has been known that coupling occurs between two closely spaced cores in a multiple core device. The coupling efficiency increases with decreasing core separation and, in the case of single-mode cores, with decreasing core diameter. There have been developed a number of couplers that are based on these principles.
Multimode and single-mode couplers have been formed by positioning a plurality of fibers side-by-side along a suitable length thereof and fusing the claddings together to secure the fibers and reduce the spacings between the cores. Coupling can be enhanced by stretching and rotating the fibers along the fused length thereof as taught in U.S. Pat. No. 4,426,215. Also, a portion of the cladding is sometimes removed by etching or grinding to decrease the intercore distance as taught in U.S. Pat. No. 4,449,781. These processes are labor intensive and do not always result in the formation of couplers exhibiting predetermined coupling characteristics. This latter mentioned disadvantage is particularly apparent in the manufacture of certain single-mode couplers wherein the coupling core sections are to remain linear.
Whereas most couplers are made by applying heat directly to the fibers to be joined, U.S. Pat. No. 3,579,316 teaches a method wherein the fibers are first inserted into a capillary tube where the ends may overlap. The capillary tube is formed of a glass having a refractive index lower than that of the fiber cladding material. Heat is applied to the capillary tube in the vicinity of fiber overlap and the tube is stretched until the diameter thereof approximates that of the original fibers. The original cores of the pulled-out part become vanishingly small, their stretched diameters being only about 1/100 the original diameters. The cladding of the original fiber becomes the core of the coupling section.
The disadvantages of the method of U.S. Pat. No. 3,579,316 are as follows. The method is costly since fibers must be inserted into a capilliary tube for each coupler. Also, the reduction in core diameter to 1/100 of the original diameter results in a lossy coupler, since the original cladding takes the place of the vanished cores. In the region of the coupler where the fiber cores taper from their "vanishingly small" size to their full size, an insufficient amount of the power can transfer from the cladding back to the core. Furthermore, it is difficult to maintain the cores straight and parallel to one another when fibers are inserted into a tube which is then stretched. Such nonlinear coupler cores can lead to decreased coupling efficiency in single-mode couplers.
Control of the excess device loss is a problem with such prior art devices. Variability of mode power in the split waveguides exists due to fabrication technique variations. In some devices, bifurcation of the input and output fiber ports is difficult to accomplish. Also many of the prior art fabrication steps are highly labor intensive and are therefore a major cost source.
Copending U.S. patent application Ser. No. 765,652 teaches an inexpensive method of making low loss couplers. In accordance with that method there is initially provided a coupler preform having a plurality of spaced glass cores extending through a matrix of glass having a refractive index lower than that of the glass cores. The coupler preform having a plurality of spaced glass cores extending through a matrix of glass having a refractive index lower than that of the glass cores. The coupler preform is heated and stretched to form a glass rod which is severed into a plurality of units. Heat is applied to the central region of each unit while the ends are pulled to elongate and taper inwardly the heated central region thereof. In one embodiment thereof the coupler comprises a matrix of etchable glass. Each of the coupler cores is surrounded by a cladding of etch-resistant glass. The ends of the coupler unit are immersed in acid to dissolve the immersed region of matrix glass from each end of the unit to expose the ends of the unit cores and the surrounding layer of etch-resistant cladding glass.
In accordance with the method of said copending application a plurality of longitudinally-extending holes are drilled in an elongated boule of matrix glass. A rod of core glass surrounded by a layer of the etch-resistant cladding glass is placed in each hole. It is difficult to drill the longitudinally extending holes parallel to each other since the drill can skew off at an angle from its intended direction. Also, drilling produces a rough surface which, if not smoothed, tends to produce seeds during the stretching operation. Furthermore, as the holes are drilled closer together to minimize the amount of matrix glass therebetween and thereby improve coupling, cracking tends to occur between holes.